In 2XXX aluminum alloys, improved strength is achievable by cold working the alloy after solution heat treatment and prior to aging. This promotes greater nucleation and refinement of the .theta.' precipitates in Al-Cu alloys, S' precipitates in Al-Cu-Mg alloys and T.sub.1 precipitates in lithium-containing Al-Cu alloys. These precipitates are believed to be responsible for imparting increased strength to the finished product.
Sheet, plate or extruded 2XXX alloy product forms can be stretched or cold rolled prior to aging to achieve the optimum distributions of .theta.', S' and/or T.sub.1 precipitation, depending upon the alloy, thereby imparting improved strength to the alloy. Products in this condition have achieved what is known in the art as a "T8" temper. However, it is quite difficult to uniformly cold work a forging through its entire thickness, in great part because such forgings typically are of non-uniform thickness and/or have a shape that does not lend itself to cold working
Fabrication of an object of complex configuration from an aluminum alloy is typically achieved by closed-die forging of a billet of the alloy to produce a near-netshape, which then requires only minimal machine finishing to achieve a final shape. The forging process permits control of metal flow, and thereby permits control of the formation of metallurgical microstructures in localized areas so that directional properties of the crystal structure of the alloy can be made to conform to directional requirements of the particular end use application of the object being fabricated
In the prior art, forging procedures for aluminum alloys were generally formulated primarily to achieve specific geometrical configurations, and were not generally designed for maximum efficiency in process scheduling or for obtaining optimum mechanical properties in the objects being fabricated. In the prior art, a forging schedule for an aluminum alloy typically included multiple forging sequences, an intermediate reheating stage, a sizing or coining operation (depending upon the size and complexity of the object being fabricated), and a final heat-treatment sequence for thermal strengthening of the object.
Forging of an aluminum alloy is conventionally performed in a die heated in the temperature range of 690.degree. F. to 880.degree. F. (366.degree.-471.degree. C.) range. If the forged alloy is of the precipitation-hardenable type such as, for example, an aluminum alloy of the 2000, 6000 or 7000 series (also known as 2XXX, 6XXX and 7XXX, respectively) as described in standard texts such as the Alcoa Aluminum Handbook, Second Edition, Aluminum Company of America, Pittsburgh, Pa. (1962), the final heat-treatment sequence conventionally requires solution heat treatment, quenching and aging of the forged object at a temperature in the 250.degree. F. to 400.degree. F. (121.degree.-204.degree. C.) range for a relatively long period of time, which typically may be from 8 to 36 hours.
Various techniques have been employed to improve the strength of 2XXX and other aluminum alloys. One study evaluated and reported on properties achieved by thermomechanically processing 2024 alloy and suggested an optimal result was achieved through the steps of solution heat treating, stress relieving by stretching and room temperature aging to the T3 temper, warm working 5-15% at 375.degree. F. (191.degree. C.) and post aging at 375.degree. F. (191.degree. C.). See Sommer, Paton and Folgner, "Effects of Thermomechanical Treatments on Aluminum Alloys," AFML-TR-72-5 (February 1972). See also Paton and Sommer, "Influence of Thermomechanical Processing Treatments on Properties of Aluminum Alloys," Paper 21, Proceedings of the 3rd International Conference on Strength of Metals and Alloys, p. 101-108, Cambridge, England (1973). Others, however, were unable to reproduce these results. See Thompson, et al., "Program to Improve the Fracture Toughness and Fatigue Resistance of Aluminum Sheet and Plate for Aircraft Applications" AFML-TR-73-247, Vol. I (September 1973). See also, Thompson, et al., "Thermomechanical Aging of Aluminum Alloys," Aluminum, Vol. 50, Part I, p. 647; Part II, p. 719 (1974).
Others have used thermomechanical processing in an effort to improve the strength of forged aluminum alloy products. U.S. Pat. No. 4,596,609 discloses a method in which an age-hardenable aluminum alloy is solution heat treated, pre-aged at a temperature below the solution heat treatment temperature for about 0.5-1.5 hours, and worked. See also Kumar, et. al., "Electron Microscopic Studies of Thermomechanically Aged 221B Aluminum Alloy," Bull. Mater. Sci., Vol. 10, No. 3, p. 217-222 (May, 1988) and Singh, et. al., "Influence of Thermomechanical Aging on Tensile Strength Properties of 2014 Aluminum Alloy," Journal of Material Science 25, p. 3894-3900 (1990).
A significant advance in the art could be realized if T8-type strength properties could be imparted to alloys which are presently not able to achieve such strength properties. More specifically, an advance in the art could be realized if T8-type strength properties could be imparted to aluminum alloys such as 2XXX and 8XXX alloys in forgings.